1. Field of the Invention
The invention is related generally to the use of resistivity measurements for evaluation of earth formations that include deep-water sediments.
2. Background of the Art
Electromagnetic induction and wave propagation logging tools are commonly used for determination of electrical properties of formations surrounding a borehole. These logging tools give measurements of apparent resistivity (or conductivity) of the formation that when properly interpreted are diagnostic of the petrophysical properties of the formation and the fluids therein.
One problem of particular interest is the evaluation of deep-water deposits in the subsurface. A cross-section of a deep-water channel sequence is shown in FIG. 3. The sequence is characterized by interbedded sands and shales with the relative thickness depending upon the relative position (horizontal and vertical) within the sequence. Prior art methods have used high resolution image logs (such as resistivity image logs) to identify channels, to use changes in dip trends and net-to-gross sand values (N/G) to identify channels and slumps. Specifically, at a location such as 201 near the center of the channel complex, the ratio N/G is large, is smaller at a location such as 202 and still smaller near the fringes of the channel at a location such as 203. Prior art methods have also used gamma ray logs to help define larger scale trends.
Image logs suffer from the drawback that they are basically limited to borehole wall observations and hence cannot “see” into the formation. This makes it difficult to evaluate the importance of thin mudstone beds within a sand layer away from the borehole. In addition, the data may be of poor quality, making it difficult to evaluate the dips of beds seen in the image. Image logs are affected by washouts and poor borehole conditions. Evaluation of the formation would be greatly improved if dips that are indicative of larger-scale trends (of the order of 2 m or more) could be evaluated. The ability to measure net to gross (N/G) ratio and evaluation of bed thickness trends is also important.
The 3DEX™ tool of Baker Atlas has a depth of investigation in the formation that is typically several meters and correspond to large-scale dip and azimuth. In contrast, the dips and azimuths from imaging devices are derived from the property (e.g., resistivity) boundaries of formation beds or laminations. When the beds or laminations are below the resolution of the imaging devices, the dips and azimuths are reliably determined. In contrast, the 3DEX measurements are sensitive to the orientation of the formation conductivity tensor. The measurements allow accurate determination of the dips and azimuths in the absence of bed boundaries, provided there exists measurable formation anisotropy. Hence, in many instances the imaging-derived dips and azimuths may be different from the 3DEX-derived ones.
U.S. patent application Ser. No. 11/740,376 of Wang et al., having the same assignee as the present application and the contents of which are incorporated herein by reference discusses in some detail the utility of multicomponent resistivity measurements, such as 3DEX™ measurements, in determination of formation dips away from the borehole and of identifying and delineating unconformities using multicomponent measurements. As noted therein, the different depths of investigation (DOI) and different vertical resolution of the multicomponent resistivity measurement and the conventional borehole imaging logs will in some circumstances result in different dips and azimuths. The borehole imaging tools usually have DOIs less than a few centimeters, whereas the multicomponent resistivity measurement reads meters into the formation. Therefore, the two measurements will read the same angles if the angles do not change significantly from the borehole. When formation angles change laterally, Wang discusses how the measurement “averaging” affects the angle data derived from 3DEX tool measurements.
U.S. Pat. No. 6,470,274 to Mollison et al. and U.S. Pat. No. 6,493,632 to Mollison et al. having the same assignee as the present application and the contents of which are fully incorporated herein by reference discloses use of a multi-component logging tool (the 3DEX tool of Baker Hughes Incorporated) for determination of anisotropic resistivity parameters of a laminated reservoir. As would be known to those versed in the art, such a laminated reservoir that has layers of different resistivities exhibits transverse isotropy even if the layers themselves are isotropic. Such a multicomponent logging tool has azimuthal sensitivity. The two Mollison patents disclose a method of analyzing data from a multicomponent logging tool to determine water saturations of the sand and shale fractions of the reservoir. The model used in Mollison assumes that the anisotropy axis is normal to the bedding plane. Similar models have been assumed in, for example, U.S. Pat. No. 6,643,589 to Zhang et al. U.S. Pat. No. 6,686,736 to Schoen et al., having the same assignee as the present invention and the contents of which are incorporated herein by reference, teaches the determination of the distribution of shales, sands and water in a reservoir including laminated shaly sands using vertical and horizontal conductivities is derived from nuclear, NMR, and multi-component induction data. The multicomponent data are inverted and an estimate of the laminated shale volume from this inversion is compared with an estimate of laminated shale volume from nuclear logs. The bulk water volume determined from the inversion is compared with a bulk irreducible water volume from NMR measurements. NMR data are then used to obtain a sand distribution in the reservoir and this sand distribution is used in a second inversion of the multicomponent data. Alternatively, a bulk permeability measurement is used as a constraint in inverting the properties of the anisotropic sand component of the reservoir. From the resistivities of the sand laminae, empirical relations are used to predict anisotropic reservoir properties of the reservoir.
The present invention is directed towards the use of 3DEX measurements to characterize geologic formations away from the borehole and/or to compare the results of this characterization with borehole imaging logs.